This invention relates to a material for a housing for shielding electronic components from external electromagnetic noise.
In recent years, digital apparatus, including microcomputers, have been developing remarkably and are used widely in office equipment, household electric appliances, automobiles, etc. In addition, microcomputers with higher integration and higher clock frequency have been increasing in number. In some cases, however, electromagnetic wave noise generated in electronic components, or the like, is carried away on signal conductors, such as flat cables, or directly affects the electronic components. Such electromagnetic wave noise brings about breakdowns, troubles, and faulty operations of the electronic components controlled and driven by weak electric current. Moreover, data recorded in highly dense recording elements of electronic components is extinguished by external magnetic fields caused by magnets, or the like, because the magnetic recording elements are made of magnetic substance with small magnetic energy.
In order to prevent these electromagnetic troubles, prior-art conductive metallic housings, such as iron housings, or synthetic resin housings containing carbon black particles as conductive filler, are utilized to shield electronic components from external electromagnetic noise.
However, each type of these housings for shielding electronic components from electromagnetic noise is imperfect and has problems.
Synthetic resin containing carbon black particles can form a light housing for shielding electronics (referred to as "C.B. resin housing"). Yet, a housing must be strong enough to house electronic components and resist external force. The housing can contain only limited carbon black particles, because the more carbon black particles contained in the synthetic resin, the weaker the synthetic resin base material becomes. Therefore, the electric resistivity given by the carbon black particles may be decreased to about 100 ohm.cm at the best, which is insufficient to shield electronic components from electromagnetic noise. In addition, since the C.B. resin is a non-magnetic substance, magnets must be attached on the outer surface of the housing. The magnets cannot shield electronics from low-frequency magnetic field perfectly, and breakdowns, faulty operations, or the like, of the electronic components are brought out.
The other prior-art housing made of iron for shielding electromagnetics (referred to as "iron housing") is bestowed with high permeability and high conductivity, or low electric resistivity of about 10.sup.-5 ohm.cm. Hence, the iron housing can prevent breakdowns or faulty operations of the electronic components by concentrating the magnetic flux of an external low-frequency magnetic field on its surface and shielding electromagnetic wave noise generated by the electronic components. However, the iron housing must be thick enough to maintain designated strength and is thus very heavy.